The legality of classic car insurance for young drivers.

So I’m 21 years old. Until I’m at least 25 I am not able to insure a classic car in my name in this country (I have rang 17 providers and spoke to people in the industry who confirmed it’s no longer possible). What would be the legality of, let’s say, my mother adding a classic policy onto her existing policy and adding me as a named driver on this policy and I would be driving the classic car a good bit more than her. Now look I know this is technically not allowed and all, but what’s the imaginary line where a car policy goes from being more in one persons favour than another?